This invention relates to an acoustic spherical lens and a method of manufacturing the same. More particularly, it relates to an acoustic spherical lens suitable for use as an acoustic wave focusing means in microscopes, especially ones utilizing high frequency acoustic energy, and to a method of manufacturing the same.
Since, in recent years, the generation and detection of high frequency acoustic waves reaching 1 GHz have become possible, the acoustic wavelength in the water has attained approximately 1 micron, and accordingly, microscopes exploiting acoustic energy have been studied.
In such apparatuses, it is important how a fine focused acoustic beam is prepared. A specific example of the prior art will be described with reference to FIG. 1. In the figure, a circular cylindrical crystal 20 of sapphire or the like has one end face which is a flat surface 21 optically polished, and the other end face which is provided with a hemispherical hole 30. A piezoelectric transducer 10 is disposed on the flat surface 21 of the crystal 20. A radio frequency signal is applied to the piezoelectric transducer 10 so as to radiate RF acoustic waves of plane waves into the crystal 20. The plane acoustic waves are focused on a predetermined focal point S by a concave lens formed by the boundary between the crystal 20 and a medium 40 as defined on the hemispherical hole 30. As is well known, when the ratio between the focal length and the numerical aperture, in other words, the F-number of the lens is sufficiently small, an extremely narrow acoustic beam can be prepared by this construction. The focused acoustic beam is subjected to disturbances such as reflection, scattering, transmission and attenuation by a specimen (not shown) located in the vicinity of the focal point. By detecting the disturbed acoustic energy, therefore, an electric signal reflective of the elastic property of the specimen can be obtained. For the detection of the acoustic energy, the foregoing crystal system may be utilized again. Alternatively, a similar crystal system may be confocally opposed and used.
As apparent from the above description, the prior art has its focusing based on the concave lens which exploits the difference of acoustic velocities in the crystal and the medium. Accordingly, in order to obtain a spherical lens having an excellent focusing property, it is essential to endow a crystal with an excellent flatness and to form a hemispherical hole of excellent sphericalness. More specifically, a spherical surface must not have an unevenness exceeding at least 1/10 of the acoustic wavelength in order to operate as the lens. This corresponds to the order of 0.1 .mu.m in case of acoustic waves at 1 GHz.
Moreover, since the attenuation of acoustic waves in the medium (usually, water) from the lens front to the focal point is very heavy, it needs to be avoided by forming a hemispherical hole of a minute numerical aperture of, for example, 0.2 mm and reducing the distance from the lens front to the focal point.
In the prior art, such a lens is machined by the polishing method. The machining based on the polishing method is an extraordinarily difficult job, and a lens with an aperture of 0.5 mm is laboriously fabricated.